Tuning apparatus for microwave resonant cavities

ABSTRACT

A coaxial electromagnetically resonant cavity comprises a tuning vane which is parallel to the axis of the cavity and extends for substantially the whole length thereof, and adjusting means for varying the distance between the tuning vane and the inner conductor of the cavity. The vane may be either conductive or dielectric material slidably mounted in the structure of the cavity. The adjusting means may include a slidably mounted wedgelike member bearing against a surface of the tuning vane. The vane may be in two parts adjustably connected together.

United States Patent Inventor Albert Henry Johnson Mudelord, Christchurch, England Appl. No. 788,417

Filed Jan. 2, 1969 Patented Apr. 6, 1971 Assignee Minister of Technology in Her Britannic Majestys Government United Kingdom, Great Britain and Northern Ireland London, England Priority Jan. 12, 1968, Feb. 23, 1968 Great Britain 1820 and 8898 TUNING APPARATUS FOR MICROWAVE RESON ANT CAVITIES 16 Claims, 4 Drawing Figs.

US. Cl 333/82, 3 33/ 83 Int. Cl 1101p 7/04 FieldotSearch 333/82(B),

82 (BT), 82, 73 (C); 334/4145; 330/56; 331/101-102; 315/3961; 333/83, 83 (A), 83 (T) [56] References Cited UNITED STATES PATENTS 2,201,326 5/1940 Trevor 333/73(C) 2,498,763 2/1950 McNall 333/83X I 2,910,659 10/1959 Caroselli 333/83X 3,140,444 7/ 1964 Carlson 334/44X FOREIGN PATENTS 214,018 3/1958 Australia 333/83 580,964 9/1946 Great Britain..... 333/83 130,561 12/1948 Australia 333/97 Primary Examinerl-1erman Karl Saalbach Assistant Examiner-Wm. H. Punter Attorney-Hall, Pollock and Vande Sande ABSTRACT: A coaxial electromagnetically resonant cavity comprises a tuning vane which is parallel to the axis of the cavity and extends for substantially the whole length thereof, and adjusting means for varying the distance between the tuning vane and the inner conductor of the cavity.

The vane may be either conductive or dielectric material slidably mounted in the structure of the cavity. The adjusting means may include a slidably mounted wedgelike member bearing against a surface of the tuning vane. The vane may be in two parts adjustably connected together.

Patented Aprifi 1971 3,573,679

2 Sheets-Sheet 1 lnven tor 7/ 2; mmpm A4.

Attorney Alba 1 y Jp/m in TUNING APPARATUS FOR MICROWAVE RESONANT CAVTTIES The present invention relates to tuning mechanisms for resonant cavities of the coaxial type, which may be used in microwave engineering apparatus for generating or for operating on high-frequency electromagnetic waves.

In this specification the term coaxial resonant cavity" should be understood as meaning any structure comprising a substantially cylindrical inner conductor and a boxlike outer conductor disposed about the inner conductor, defining a space around the inner conductor within which electromagnetic waves at one or more resonance frequencies can form standing wave patterns.

The inner conductor or the outer conductor may be discontinuous. The discontinuity may form part of an electronic device, or an electronic device may be connected across the discontinuity. The outer conductor may be a right circular cylinder, or alternatively it may be in the form of a box of square or rectangular cross section. Known methods of fine tuning such cavity resonators have various disadvantages. Usually an end plate joining the inner conductor to the outer conductor, or two opposite sides of a box section outer conductor, are slidably mounted for tuning purposes. However, this form of tuning requires the conduction of large radiofrequency currents through sliding contacts. It is found that sliding contacts soon tend to become unsatisfactory for carrying large radiofrequency currents, and create electrical noise, especially when they are used for carrying currents at very high frequencies. Moreover, the radiofrequency losses caused by the imperfections of the sliding contacts undesirably reduce the Q factor of the resonant cavity. Another method is to provide a localized variable capacitance, by providing either a conductive radial probe protruding from the outer conductor an adjustable distance towards the inner conductor, or an annulus of high-permittivity material which has a greater capacitive effect when it is brought near to a voltage node in the standing-wave pattern formed within the cavity. However, the use of localized capacitive loading devices tends to promote parasitic resonances.

it is an object of the invention to provide a coaxial resonant cavity with an alternative tuning arrangement, suitable for use at very high microwave frequencies.

According to the present invention, a tuning mechanism in a coaxial cavity resonator, as hereinbefore defined, includes a tuning vane or plate extending parallel to the axis of the cavity for substantially the whole length of the cavity and adjusting means for varying the distance between the tuning vane and the inner conductor of the cavity. The tuning vane may either be conductive, or alternatively of a dielectric material having a dielectric constant considerably greater than that of air.

The tuning vane may be slidably mounted in the cavity, and may protrude through the outer conductor of the cavity. It may have an outer edge outside the cavity disposed at an angle to the axis of the cavity, and the adjusting means may include a wedge-shaped member slidably mounted between the outer edge of the vane and a bearing surface which is fixed with respect to the the cavity.

The adjusting means may include an adjusting member moveable along an axis intersecting the plane of the tuning vane and cooperating with the tuning vane to move it towards or away from the inner conductor of the cavity.

The tuning vane may be made in two parts, comprising a tuning part which protrudes into the cavity and a bearing part which bears on the adjusting member and is adjustably connected to the tuning part. The tuning part may be either conductive or alternatively of a dielectric material having a dielectric constant considerably greater than that of air.

The two-part construction of the tuning vane allows the range of frequencies attainable to be varied, and the transverse arrangement of the adjusting member leads to a convenient and compact mechanical arrangement.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

FIG. 1 is a transverse section and FIG. 2 is a longitudinal section of a cavity resonator in a Gunn-effect oscillator apparatus. The section planes of FIG. 1 and FIG. 2 are shown by the line H in FIG. 2 and the line [MI in FIG. 1 respectively and HG. 3 is a transverse section and FIG. 4 is a longitudinal section of another cavity resonator in a Gunn-effect oscillator. The section planes of HO. 3 and FIG. 4 are shown by the line lll-lll in FIG. 4 and the line lV-lV in FIG. 3 respectively.

These drawings are not necessarily drawn to scale.

FlGS. 1 and 2 show a cavity resonator l of circular cross section, formed in a block 2 between end plates 3 and 4. An inner conductor 5 is mounted on the end plate 3 coaxially within the resonator 1. A Gunn diode 6 is mounted on an adjusting screw 7 so that one of its electrodes is in electrical contact with the said adjusting screw. The other electrode of the Gunn diode is placed in electrical contact with the inner conductor 5 by adjusting the screw 7. An output line 8 is connected between the inner conductor 5 and a coaxial output plug 9. A longitudinal conductive vane 10 is slidably mounted in a slot 11 in the block 2. A sliding member 12 slides in the slot 11 between the conductive vane 10 and a plate 13 which is rigidly attached to the block 2 by screws 14. The sliding member 12 and the conductive vane bear on each other at surfaces 15 which are inclined at an acute angle to the axis of the cavity so that longitudinal movement of the member 12 produces radial movement of the conductive vane 10. An adjusting screw 16 is threaded through the end plate 4 and bears on the sliding member 12. The sliding member 12 is spring loaded against the adjusting screw 16 by the spring 17. Two springs 18 (shown dashed) are arranged to pull the conductive vane 10 against the sliding member 12. A spring loaded plunger 19 having a conductive tip 20 presses the conductive vane 10 against one wall of the slot 11. Screw 21 is provided for adjusting the compression of the spring on the spring loaded plunger 19. A screw 22 is threaded through the end plate 4 opposite to the inner conductor 5. A gap of adjustable width d is left between the screw 22 and the end of the inner conductor 5. A locking nut 23 is provided to prevent undesired movement of the screw 22.

The part of the cavity resonator within the block 2 and around the inner conductor 5 forms a section of coaxial transmission line, of length y, short-circuited at one end by the plate 3. The length y is less than a quarter of a wavelength and the section therefore presents an inductive reactance X L at its end nearest the end plate 4, at the operating frequency of the oscillator. The gap between the end of the inner conductor 5 and the screw 22 provides a capacitive reactance X which cooperates with the inductive reactance of the transmission line section to give an effect comparable to that of a parallel tuned circuit, with a resonant frequency determined by the reactances.

A coarse control of the resonant frequency is obtained by adjustment of the screw 22, which alters the distance d and thereby varies the capacitive reactance X The lock nut 23-is then tightened.

The inductive reactance X L depends on the characteristic impedance Z0 and the length of y of the transmission line section, according to the well-known equation X Z0 tan where A is the wavelength in the transmission line at the operating frequency. The characteristic impedance Z0 of a simple air-dielectric coaxial transmission line is given by the equation Zu=l 38 log (11/0) (2) that given the the equation (2). Fine tuning is achieved by radial adjustment of the conductive vane 10, which varies the effective characteristic impedance of the line and thereby adjusts the inductive reactance X The radial adjustment of the conductive vane is accomplished by adjustment of the screw 16. Turning the screw 16 causes longitudinal motion of the sliding member 12. When the screw 16 is turned in one direction this longitudinal motion will cause the conductive vane 10 to slide radially in the slot 11 towards the center conductor 5. The radial motion of the conductive vane 10 will be reversed when the screw 16 is turned in the other direction. This is so because the springs 18 pull the conductive vane 10 outwards towards the plate 13. The slot 11 and the conductive vane 10 extend to the full length of the coaxial resonant cavity I and the bearing surfaces are arranged that the face of the conductive vane 10 nearest to the center conductor 5 always remains parallel to the center conductor 5. The loading which the conductive vane 10 presents to the coaxial resonant cavity I is therefore uniformly distributed over its entire length. The conductive vane 10 is pressed firmly against the walls of the slot 11 by the spring loaded plunger 19 and conductive tip to ensure good electrical contact between the said conductive vane 10 and the outer conductor formed by a machined surface in the block 2. The block 2 and the end plates 3 and 4 are made of highly conductive metal.

Gunn diode oscillators are now well known and their operation need not be explained in detail here. The oscillator comprising the resonant cavity of FIGS. 1 and 2 may be put into operation by supplying the Gunn diode with power from a power source (not shown) which is controlled so that the Gunn diode operates over a suitable part of its voltage/current characteristic. Electromagnetic energy at the resonant frequency of the cavity, may then be extracted through the output line 8 and the coaxial output plug 9.

The currents induced in the conductive vane 10 by the electric and magnetic fields are small compared with the total oscillatory current. Therefore any imperfections in the metal to metal contacts between the conductive vane 10 and the walls of the slot 11 are less deleterious than if such contacts were in regions of high current density. Hence the Q factor of the resonant cavity is maintained at a high value.

As the conductive vane 10 slides along the slot 11 its frontal area always remains parallel to the center conductor 5. The change in capacity between the conductive vane 10 and the inner conductor 5 is a fairly linear function of the distance travelled by the conductive vane. Hence the relationship between the resonant frequency and the rotation of the adjusting screw 16, is not inconveniently nonlinear. The loading effect, which the conductive vane 10 has on the coaxial resonant cavity 1, is uniformly distributed over its entire length. This distributed loading does not produce the unwanted resonances which are an undesirable characteristic of some other methods of tuning. The present method of tuning is effected with a mechanical ease which is extremely difficult to achieve when employing moving parts with sliding joints which carry the whole of the oscillatory current.

An alternative embodiment of the invention will now be described, by way of example only, with reference to FIGS. 3 and 4.

Parts of this embodiment which correspond to similar or equivalent parts in the embodiment of FIGS. 1 and 2 are given the same references.

FIGS. 3 and 4 show a cavity resonator l of circular cross section, fonned in a block 2. An inner conductor 5 is mounted in the block 2 coaxially within the resonator 1. A Gunn diode 6 is mounted on an adjusting screw 7 so that one of its electrodes is in electrical contact with the said adjusting screw. The other electrode of the Gunn diode is placed in electrical contact with the inner conductor 5 by adjusting the screw 7. An output line 8 is connected between the inner conductor 5 and a coaxial output plug 9. A longitudinal conductive tuning vane 10, part of which has been cut away is slidably mounted in a slot 11 in the block 2 and may protrude into the cavity 1.

The end of the conductive vane 10 which is remote from the cavity 1 has a threaded nut attachment 29 which is a sliding fit in a suitable hole in the block 2. A movable slider plate 10a fits inside the cutaway portion of the longitudinal conductive vane 10. An adjusting screw 30 cooperates with the threaded hole in the nut 29 and bears on that end of the slider plate 10a which is remote from the cavity 1. Two leaf springs 31 urge the longitudinal conductive vane 10, together with the slider plate 10a and adjusting screw 30, along the slot 11 and radially into the cavity 1 toward the center conductor 5. Spring loaded plungers 19 having conductive tips 20 press the conductive vane 10 against one wall of the slot 11. The screws 21 are provided for adjusting the compression of the spring on the spring loaded plungers 19. A screw 22 is threaded through the block 2 opposite to the inner conductor 5. A gap of adjustable width d is left between the screw 22 and the end of the inner conductor 5. A locking nut 23 is provided to prevent undesired movement of the screw 22. A cone-shaped member 32 is rotatably mounted on bush bearings 33 in a cavity 34 of the block 2. The conical member 32 also protrudes through the cutaway portion of the conductive vane 10. The conical member 32 has a threaded shaft 35 at one end which cooperates with a threaded hole in the block 2. The conical member 32 has also a transverse slot 36 in the end remote from the shaft 35. A control spindle 37 is also mounted in the cavity 34 and retained in the cavity by the retaining springs 38. The control spindle 37 has a tongue 39 which engages the slot 36 of the conical member 32. The conical member 32 is spring loaded toward the control spindle 37 by a spring 40. The slider plate 10a has a V-shaped portion 41 which bears against the conical surface of the conical member 32.

Radial adjustment of the conductive vane 10 into and out of the cavity 1 is effected by rotating the control spindle 37. The control spindle 37 drives the conical member 32 and attached shaft 35 by means of the tongue 39 in the slot 36. Hence the rotation of the spindle 37 screws the shaft 35 up or down in the threaded hole in which it is engaged, thereby altering the position of the conical member 32 and of the slider plate 10a. Consequently the position of the turning vane 10 is altered because the leaf springs 31 urge the V shaped jaws of the slider plate 10a against the conical surface. The spring loading of the tuning vane 10 must be sufficient to overcome the frictional resistance to its movement by the bearing surfaces of the slot 11 in which it slides and the resistance caused by the pressure of the spring loaded plungers 19.

In operation the conical member 32 may be positioned initially so that it is in the center of its limits of travel. This is indicated approximately in the drawing of FIG. 3. The adjusting screw 30 may then be adjusted to set the mean distance to which the conductive tuning vane 10 will travel. Rotation of the control spindle 37 in one sense will cause the conductive vane 10 to slide along the slot in one direction while rotation in the opposite sense will cause the conductive vane to slide in the other direction.

In a typical application at wavelengths of the order of 3 centimeters or less, the range of movement of the conductive vane may be less than fifty-thousandths of an inch.

In alternative embodiments of the invention the tuning vane may be of a material having a relatively higher dielectric constant than air. The effect of moving such a vane into the resonant cavity is similar to that hereinbefore described, as it caused the capacitance between the inner and outer conductors to be modified.

I claim:

1. An electromagnetically resonant cavity structure as hereinbefore defined comprising an inner conductor, an outer conductor having a longitudinal slot therein, tuning vane means moveably mounted in said slot and extending for substantially the whole length of the said resonant cavity and adjusting means mounted on the said resonant cavity structure and bearing on the said tuning vane for altering the distance between the said tuning vane and the said inner conductor.

2. A structure as'claimed in claim 1 and wherein the tuning vane has an outer edge outside the resonant cavity, the said outer conductor has a bearing surface rigidly attached to or formed on it and disposed at an acute angle to the said outer edge of the tuning vane and the adjusting means comprises a wedge-shaped member slideably mounted between the said outer edge and the said bearing surface.

3. A structure as claimed in claim 2 and wherein the adjusting means comprises screw means rotatably mounted on the cavity structure and bearing against the said wedge-shaped member for sliding the said member along the said fixed hearing surface, and spring means mounted on the said cavity structure for urging said tuning vane against said wedgeshaped member.

4. A structure as claimed in claim 2 and wherein the tuning vane is made of a dielectric material.

5. A structure as claimed in claim 2 and wherein the tuning vane is made of a metallic material.

6. A structure as claimed in claim 5 and wherein there is provided adjustable spring loaded plunger means mounted on the said cavity structure and bearing on said tuning vane to press said vane against the cavity structure.

7. A structure as claimed in claim 1 and wherein the tuning vane is made of a dielectric material.

8. A structure as claimed in claim I and wherein the tuning vane is made of a metallic material.

9. A structure as claimed in claim 8 and wherein there is provided adjustable spring loaded plunger means mounted on the said cavity structure and bearing on said tuning vane to press said vane against the cavity structure.

10. Tuning apparatus as claimed in claim 1 and wherein the adjusting means includes an adjusting member mounted on the cavity structure and movable along an axis intersecting the plane of the said tuning vane, said adjusting member cooperav ing with the said tuning vane to adjust its distance from the inner conductor of the cavity.

11. A structure as claimed in claim 10 comprising screw means for causing the adjusting member to move along the said axis and wherein the adjusting member has a bearing surface inclined to the said axis.

12. A structure as claimed in claim 11 and wherein the hearing surface is of conical form.

13, A structure as claimed in claim 10 and wherein the said tuning vane comprises a tuning part which protrudes into the cavity and a bearing part which bears on the said adjusting member, said bearing part being adjustably connected to said tuning part.

14. Tuning apparatus as claimed in claim 11 and wherein the tuning part is made of a dielectric material.

15. Tuning apparatus as claimed in claim 11 and wherein the tuning part is made of a metallic material.

16. Tuning apparatus as claimed in claim 15 and wherein there is provided adjustable spring loaded plunger means mounted on the cavity structure and bearing on the said tuning vane to press said tuning vane against the said cavity structure. 

1. An electromagnetically resonant cavity structure as hereinbefore defined comprising an inner conductor, an outer conductor having a longitudinal slot therein, tuning vane means moveably mounted in said slot and extending for substantially the whole length of the said resonant cavity and adjusting means mounted on the said resonant cavity structure and bearing on the said tuning vane for altering the distance between the said tuning vane and the said inner conductor.
 2. A structure as claimed in claim 1 and wherein the tuning vane has an outer edge outside the resonant cavity, the said outer conductor has a bearing surface rigidly attached to or formed on it and disposed at an acute angle to the said outer edge of the tuning vane and the adjusting means comprises a wedge-shaped member slideably mounted between the said outer edge and the said bearing surface.
 3. A structure as claimed in claim 2 and wherein the adjusting means comprises screw means rotatably mounted on the cavity structure and bearing against the said wedge-shaped member for sliding the said member along the said fixed bearing surface, and spring means mounted on the said cavity structure for urging said tuning vane against said wedge-shaped member.
 4. A structure as claimed in claim 2 and wherein the tuning vane is made of a dielectric material.
 5. A structure as claimed in claim 2 and wherein the tuning vane is made of a metallic material.
 6. A structure as claimed in claim 5 and wherein there is provided adjustable spring loaded plunger means mounted on the said cavity structure and bearing on said tuning vane to press said vane against the cavity structure.
 7. A structure as claimed in claim 1 and wherein the tuning vane is made of a dielectric material.
 8. A structure as claimed in claim 1 and wherein the tuning vane is made of a metallic material.
 9. A structure as claimed in claim 8 and wherein there is provided adjustable spring loaded plunger means mounted on the said cavity structure and bearing on said tuning vane to press said vane against the cavity structure.
 10. Tuning appAratus as claimed in claim 1 and wherein the adjusting means includes an adjusting member mounted on the cavity structure and movable along an axis intersecting the plane of the said tuning vane, said adjusting member cooperating with the said tuning vane to adjust its distance from the inner conductor of the cavity.
 11. A structure as claimed in claim 10 comprising screw means for causing the adjusting member to move along the said axis and wherein the adjusting member has a bearing surface inclined to the said axis.
 12. A structure as claimed in claim 11 and wherein the bearing surface is of conical form.
 13. A structure as claimed in claim 10 and wherein the said tuning vane comprises a tuning part which protrudes into the cavity and a bearing part which bears on the said adjusting member, said bearing part being adjustably connected to said tuning part.
 14. Tuning apparatus as claimed in claim 11 and wherein the tuning part is made of a dielectric material.
 15. Tuning apparatus as claimed in claim 11 and wherein the tuning part is made of a metallic material.
 16. Tuning apparatus as claimed in claim 15 and wherein there is provided adjustable spring loaded plunger means mounted on the cavity structure and bearing on the said tuning vane to press said tuning vane against the said cavity structure. 